Siderophores are secondary metabolites produced by microorganisms to acquire Fe(III) from the aerobic environment. The ferric siderophore complexes are transported into the cells by a high affinity and specific uptake system which uses several membrane proteins. In Gram-negative bacteria the outer membrane receptor proteins give this process specificity. E. coli has an outer membrane (OM) receptor protein (FepA) for the siderophore it produces (enterobactin), but it also has OM receptor proteins for Fe(III) siderophore complexes it does not produce; for instance, FhuA for ferrichrome and FecA for Fe(III) citrate. The main goal for the present application is to determine the structures of these receptor proteins and their complexes with cognate Fe(III) siderophore by single crystal X-ray diffraction. It has been shown that solubilized FepA, once purified, retains its specificity and is in a conformation closely similar to or the same as the one it has in the membrane. The FepA protein has been crystallized and shows diffraction to 5.5 A (rotating anode) and 3.3 A (synchrotron) with space group P21, using thin (50y) plates. It appears practical to obtain the structure at 3 A resolution with slightly thicker crystals. A structure determination is also planned for the complex FepA.Fe(III) enterobactin. Both structures will give significant information about the specificity and mechanism of transport and also about membrane transport in general. The structural information will also be correlated with kinetic and thermodynamic studies using the protein. Similar single crystal structure determinations and physical studies are planned on FhuA and its complex with ferrichrome, FecA, and PupA (from Pseudomonas WCS 358). The project also proposes structure determinations of new siderophores and synthetic ligands specific for Fe(III). The latter are important as orally active agents to treat iron overload, and possibly other diseases which involve iron, such as malaria.